How to get a reference to a 'dynamic' object call?

Question

Well, my question is not very clear, but what I want to do is the following:

average = [1, 2, 3].inject(0) { |sum, el| sum + el } / this.size

The code above won't work because of the ridiculous call to this, But what I want to accomplish is to get a reference to the array to which I'm calling inject on. (in this case [1, 2, 3]), Given my ignorance in ruby I did it with this. But, Could you please tell me how to do it right? Is it possible at all to do it without a variable reference?

Thanks in advance!

Probably related: http://stackoverflow.com/questions/4341161/is-there-a-ruby-method-that-just-returns-the-value-of-a-block — Andrew Grimm, Sep 06 '11 at 23:08

James Kyburz · Accepted Answer · 2011-09-02T14:21:21.310

4

There is no this in ruby the nearest thing is self.

Here are some examples to help you on your way

#example 1 not self needed numbers is the array

numbers = [1, 2, 3]

numbers.reduce(:+).to_f / numbers.size

# example 2 using tap which gives access to self and returns self
# hence why total variable is needed

total = 0
[1, 2, 3].tap {|a| total = a.reduce(:+).to_f / a.size }

# instance_eval hack which accesses self, and the block after do is an expression 
# can return the average without an extra variable

[1, 2, 3].instance_eval { self.reduce(:+).to_f / self.size } # => 2.0

So far I prefer example 1

edited Sep 02 '11 at 14:21

answered Sep 02 '11 at 14:07

James Kyburz

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Yep... but he would like to it without a variable reference. It's the main requirement... – lucapette Sep 02 '11 at 14:08
lucapette is right. I'm playing with Ruby to get better at it, and I wondered If I could do it, in the way I described. thanks ! – jlstr Sep 02 '11 at 14:10
Well, in your latest case, the expression returns an array, instead of the float value of an average calculation. – jlstr Sep 02 '11 at 14:14
user766388 yes but it sets total too – James Kyburz Sep 02 '11 at 14:16
Your last example is as close as it gets, so, kudos for that one, and thanks for the lesson Mr. Kyburz! – jlstr Sep 02 '11 at 14:28
2

If you are really a Ruby beginner, there's no point trying to learn tricky code like this, IMHO – John Sep 02 '11 at 14:28
Please do explain Why John? I've been asked to do tricky things in Ruby interviews. However you seem to know where the line is drawn between what's useful and what's pointless. – jlstr Sep 02 '11 at 14:48
You don't need to specify `self.` in your last snippet, and it's not hackish IMO. – Mladen Jablanović Sep 02 '11 at 16:18
Mladen Jablanović agreed, it was to more to show what self is – James Kyburz Sep 02 '11 at 17:56

score 0 · Answer 2 · answered Sep 02 '11 at 14:16

'this' doesn't refer to the array, so that won't work at all. I don't think it's possible to get a reference to the array as you have declared it. But you will never run into such a problem, because if the array is hardwired into the code like that, then the divisor might as well be too, so you can just write '/3' at the end.

In the general case where you have an array of unknown size, you would also have a name for the array, so you could use that. Like:

a = [1, 2, 3]
average = a.inject(0) {|sum, el| sum+el} / a.size

So, the short answer is that ruby doesn't have such capability? And I agree that I'm certainly not going to need it anyways. I just wanted to know if the language had a syntax for it. — jlstr, Sep 02 '11 at 14:26

How to get a reference to a 'dynamic' object call?

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